1. Field of Invention
The present invention relates generally to structures for positioning one or more diagnostic or therapeutic elements within the body and, more particularly, to power control systems for use with the same.
2. Description of the Related Art
There are many instances where diagnostic and therapeutic elements must be inserted into the body. One instance involves the treatment of cardiac conditions such as atrial fibrillation and atrial flutter which lead to an unpleasant, irregular heart beat, called arrhythmia.
Normal sinus rhythm of the heart begins with the sinoatrial node (or xe2x80x9cSA nodexe2x80x9d) generating an electrical impulse. The impulse usually propagates uniformly across the right and left atria and the atrial septum to the atrioventrcular node (or xe2x80x9cAV nodexe2x80x9d). This propagation causes the atria to contract in an organized way to transport blood from the atria to the ventricles, and to provide timed stimulation of the ventricles. The AV node regulates the propagation delay to the atrioventricular bundle (or xe2x80x9cHISxe2x80x9d bundle). This coordination of the electrical activity of the heart causes atrial systole during ventricular diastole. This, in turn, improves the mechanical function of the heart. Atrial fibrillation occurs when anatomical obstacles in the heart disrupt the normally uniform propagation of electrical impulses in the atria. These anatomical obstacles (called xe2x80x9cconduction blocksxe2x80x9d) can cause the electrical impulse to degenerate into several circular wavelets that circulate about the obstacles. These wavelets, called xe2x80x9creentry circuits,xe2x80x9d disrupt the normally uniform activation of the left and right atria.
Because of a loss of atrioventricular synchrony, the people who suffer from atrial fibrillation and flutter also suffer the consequences of impaired hemodynamics and loss of cardiac efficiency. They are also at greater risk of stroke and other thromboembolic complications because of loss of effective contraction and atrial stasis.
Although pharmacological treatment is available for atrial fibrillation and flutter, the treatment is far from perfect. Many believe that the only way to treat the detrimental results of atrial fibrillation and flutter is to actively interrupt all of the potential pathways for atrial reentry circuits.
One surgical method of treating atrial fibrillation by interrupting pathways for reentry circuits is the so-called xe2x80x9cmaze procedurexe2x80x9d which relies on a prescribed pattern of incisions to anatomically create a convoluted path, or maze, for electrical propagation within the left and right atria. The incisions direct the electrical impulse from the SA node along a specified route through all regions of both atria, causing uniform contraction required for normal atrial transport function. The incisions finally direct the impulse to the AV node to activate the ventricles, restoring normal atrioventricular synchrony. The incisions are also carefully placed to interrupt the conduction routes of the most common reentry circuits. The maze procedure has been found very effective in curing atrial fibrillation. However, the maze procedure is technically difficult to do. It also requires open heart surgery and is very expensive. Thus, despite its considerable clinical success, only a few maze procedures are done each year.
More recently, maze-like procedures have been developed utilizing catheters and probes which can form lesions on the endocardium to effectively create a maze for electrical conduction in a predetermined path. Exemplary catheters are disclosed in commonly assigned U.S. Pat. No. 5,582,609. Exemplary surgical soft tissue coagulation probes employing a relatively shorter and stiffer shaft than a typical catheter are disclosed in commonly assigned U.S. patent application Ser. No. 08/949,117, filed Oct. 10, 1997, and U.S. patent application Ser. No. 09/072,835, filed May 5, 1998, both of which are incorporated by reference. Such probes may, for example, be used to treat atrial fibrillation in procedures wherein access to the heart is obtained by way of a thoracostomy, thoracotomy or median stemotomy.
Typically, the lesions are formed by ablating tissue with an electrode carried by the catheter or ablation probe. Electromagnetic radio frequency (xe2x80x9cRFxe2x80x9d) energy applied by the electrode heats, and eventually kills (or xe2x80x9cablatesxe2x80x9d) the tissue to form a lesion. During the ablation of soft tissue (i.e. tissue other than blood, bone and connective tissue), tissue coagulation occurs and it is the coagulation that kills the tissue. Thus, references to the ablation of soft tissue are necessarily references to soft tissue coagulation. xe2x80x9cTissue coagulationxe2x80x9d is the process of denaturing proteins in tissue and heating the fluid within the tissue cell membranes which causes it to jell, thereby killing the tissue.
A primary goal of many soft tissue coagulation procedures is to create contiguous lesions (often long, curvilinear lesions) without over-heating tissue and causing coagulum and charring. Soft tissue coagulation occurs at 50xc2x0 C., while over-heating occurs at 100xc2x0 C. A problem in the related art is the issue of rapid turning on and off power when a coagulation electrode loses contact with tissue. Tissue in contact with a coagulation electrode acts as a load to the power circuit powering the electrode, usually an RF power circuit. When the coagulation electrode is pulled away from tissue or efficacious contact is lost, the load is removed, and the voltage output of the power circuit may change. Voltage may rise suddenly, which can cause problems when the electrode is reintroduced into contact with tissue, such as arcing or charring. As a safety consideration, the circuit in conventional systems is powered off for a predetermined period by turning off the power to the RF coagulation electrode when contact is lost.
However, the inventors herein have determined that powering a circuit completely off can result in a number of problems. For example, abrupt powering on of a coagulation electrode can char tissue if the voltage rise is too rapid. Additionally, powering a circuit completely off introduces the delay associated with powering the circuit back on into the procedure. Not only is this delay inconvenient, it can also be detrimental to the patient, especially since the loss of contact can happen many times during a procedure. For example, soft tissue coagulation probes can be used to perform a maze procedure during a mitral valve replacement, which requires cardiopulmonary bypass. The longer the patient is on bypass, the greater the likelihood of morbidity and mortality. Consequently, there is a need to quickly recover from a loss of electrode-tissue contact, without completely shutting off the power supply.
Another problem identified by the present inventors has been verifying that tissue is in contact with a coagulation electrode prior to or during a surgical or catheter-based procedure, which is generally termed electrode contact verification. This is a problem pervasive throughout all surgery being performed remotely, especially when direct visual line-of-sight is not present. The use of fluoroscopic techniques is somewhat inaccurate, and requires the use of human feedback. Accordingly, a need exists for an automated control system for electrode contact verification, and optionally with visual and/or audio feedback when there is loss of contact between the electrode and tissue.
Yet another problem identified by the present inventors is associated with tissue treatment efficacy when coagulating tissue. Specifically, because different tissues in the human body and between patients absorb energy at different rates, it is difficult to ascertain when proper tissue coagulation has been completed. Heretofore, ad hoc techniques have been used to determine when the soft tissue coagulation process has been completed. One technique is visual inspection. Another is applying coagulation energy for a predetermined period based on an estimate of the amount of time required to produce a therapeutic lesion. Such techniques are not always as reliable as desired. Thus, there is a need for accurately determining when tissue has been properly coagulated, so that coagulation may be automatically stopped.
Accordingly, the object of the present invention is to provide an apparatus for positioning an operative element (such as a coagulation electrode) within the body that avoids the aforementioned problems. Other operative elements include lumens for chemical or cryogenic coagulation, laser arrays, ultrasonic transducers, microwave electrodes, and D.C. hot wires.
In accordance with one advantageous aspect of a present invention, a power control system is provided which, in response to an indication that mechanical or efficacious contact between an electrode and tissue has been lost, will merely reduce the power to the electrode, rather than completely shut it off. In one embodiment, this is accomplished by ramping down the amplitude of a RF waveform supplied to the electrode to a lower level. Adjustment of the RF signal amplitude need not involve shutting off the RF power supply.
One exemplary method of determining when contact is lost involves the use of impedance measurements. Here, the impedance is measured and compared to an expected impedance. When the impedance is greater than the expected impedance, the RF source is driven to a relatively low voltage level, such as 5V, that allows safe continued operation and impedance measurement. When the measured impedance is less than the expected impedance, the RF source ramps up to the levels needed to reach the set temperature.
There are a number of advantages associated with such a system. For example, merely reducing voltage, as opposed to shutting it off completely, increases the speed at which probe-based maze procedures proceed by reducing the down time resulting from a loss of mechanical or efficacious electrode-tissue contact. In addition, when tissue contact resumes, voltage is ramped back up. As such, tissue damage due to an abrupt voltage rise is avoided.
In accordance with another present invention, a control system is provided which verifies electrode-tissue contact. Preferably, data are collected relating to variables that can be used by a processor for electrode contact verification. The data, which are sampled by the controller before and/or during the soft tissue coagulation process, may be temperature or tissue impedance data. For example, a rise in temperature over a predetermined period of time usually means that the electrode is in contact with tissue and is heating tissue rather than blood. Such a temperature rise may be measured prior to coagulation by applying a small amount of energy (less than that required for coagulation) to the tissue to verify contact. Conversely, a drop-off in temperature during coagulation may mean contact has been lost. In the case of impedance, a flat profile of impedance over frequency indicates that there is no tissue-electrode contact.
When it is determined from the data that efficacious electrode-tissue contact has been lost, the processor instructs the console to sound a visual or audio alarm. Voltage may also be ramped down as described above. In addition to the advantages related to power control and audio/visual feedback, this invention also reduces reliance on fluoroscopic techniques.
In accordance with another present invention, a control system is provided which determines when the soft tissue coagulation process is completed. In one embodiment, tissue impedance measurements are used to determine efficacious lesion formation. A change in the impedance versus frequency curve, from a sloping curve to a flat curve, indicates that tissue coagulation is completed. Temperature can also be used. Specifically, for a given coagulation energy level and time period, a predetermined temperature profile over time indicates that a lesion has been formed. As a result, coagulation procedures, especially those involving the formation of multiple therapeutic lesions, may be performed more efficiently.
In accordance with still another present invention, a control system is provided which brings the temperature at the electrode to a temperature that is less than the maximum set temperature, maintains that temperature at the electrode at this temperature for a predetermined period, and then increases the temperature at the electrode to the set temperature In a preferred embodiment, the temperature at which tissue is maintained prior to ramping up may only be sufficient to create a transmural lesion in a relatively thin anatomical structure, while the set temperature is sufficient to create a transmural lesion in a relatively thick structure. Such a control system provides a number of important benefits. For example, in those instances where the tissue structure turns out to be relatively thin, a transmural lesion may be completed (and power delivery stopped) before the temperature reaches the set temperature because lower temperatures will be automatically used prior to reaching the set temperature. In other words, the system automatically attempts to form a lesion at a lower temperature before ramping up to the higher temperature. As many lesions will be formed at the lower temperature, coagulation procedures performed using the present control system are less likely to cause tissue charring and coagulum formation than procedures performed with conventional control systems.
The variable temperature set point system described in the preceding paragraph is also useful in epicardial applications where electrodes are placed on the epicardial surface of a heart chamber. Here, blood flow within the heart chamber produces a convective cooling effect on the heart surface and makes the creation of transmural lesions from the epicardial surface more difficult. As a result, higher temperatures (measured at the electrodes) or increased energy delivery duration is required to create a transmural lesion. Ramping the temperature to a temperature that is less than the maximum set temperature and maintaining that temperature for a predetermined period causes desiccation of the epicardial tissue and improves electrode/tissue contact. Then, when the temperature is increased to the maximum set temperature, tissue vaporization is less likely because the tissue is already desiccated. Conversely, when the temperature of tissue that has not been desiccated is suddenly increased from body temperature to the maximum set temperature required to make transmural lesions on the epicardial surface, vaporization commonly occurs. This can lead to perforation of the myocardium or the dislodgment of tissue.
Tissue coagulation depth can also be controlled by the varying the length of RF delivery. Longer RF applications usually produce deeper tissue coagulation.
In accordance with still another present invention, an interface is provided which audibly or visually indicates the status of the various aspects of the system such as, for example, ablation power, temperature, tissue/electrode contact, tissue impedance, time elapsed, type of electrode, and type of probe. In a preferred embodiment, the console may be driven by software that is modular and upgradeable to allow for new parameters to be displayed and monitored.
The above described and many other features and attendant advantages of the present invention will become apparent as the invention becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings.